1. Field of the Invention
The present invention relates, in general, to a head-mounted display and, more particularly, to an optical system for a head-mounted display, in which lenses are arranged along an optical axis to allow angles of principal rays incident upon an image plane to be gradually increased, thereby increasing an angle of view, reducing a length of the optical system, and increasing the sense of presence and the degree of involvement
2. Description of the Related Art
Generally, a head-mounted display (HMD) refers to a device that is worn on the head of a user, has an optical system embedded therein, and enables the user to enjoy large images in front of his or her eyes.
Particularly, the HMD is realized with both eyes of a user isolated from his or her surroundings, is provided therein with optical lenses, enables the user to enjoy images on a considerably wider screen than a real screen, and allows the degree of involvement, the sense of presence, and the degree of perception to be further increased.
The HMD has been provided in various shapes such as glasses, a helmet, and a cap. Recently, in consideration of a tendency toward popularization based on demands of consumers and technical development, in addition to portability and comfort, studies have been actively made of designs for small-sized, lightweight, and simple and elegant designs.
The basic principle of the HMD places an object within a focal length of a concave mirror so that an upright virtual image is enlarged and formed behind the concave mirror, and enables a user to recognize the enlarged virtual image so as to enjoy the enlarged images.
FIG. 1 schematically shows an optical system provided for a conventional head-mounted display.
As shown in FIG. 1, a liquid crystal display (LCD) on which an image input outside is displayed is located at one side, and incident light of the image is reflected on a half mirror, is incident on an concave mirror, so that a user can appreciate an upright virtual image enlarged behind the concave mirror.
The prior art using this principle is disclosed in Korean Patent Nos. 10-0304622 and 10-0272375, and Korean Unexamined Patent Application Publication No. 10-2005-0005823.
First, in the first technology (Korean Patent No. 10-0304622), an optical system for a head-mounted display in which an image produced from a display device is enlarged and formed by light irradiated by a light source and can be provided to a user, includes an imaging lens unit that has at least one lens formed of a high-refraction high-dispersion material and having strong negative power and at least one lens formed of a low-refraction low-dispersion material and having positive power and which condenses the image produced from the display device, a filter lens that has positive power and forms an incident image on a primary imaging plane along with the imaging lens unit, a half mirror that transmits or reflects incident light to change its path, and a reflective mirror on which light split and cast from the half mirror and which reflects the light toward the eyes of the user.
The second technology (Korean Patent No. 10-0272375) includes an optical image generating means of irradiating image information input from the outside, a reflecting and transmitting means of reflecting and transmitting an optical image from the optical image generating means, and a virtual image forming means of forming the optical image reflected by the reflecting and transmitting means into an upright virtual image and being asymmetrically installed at a position corresponding to the reflecting and transmitting means.
Finally, in the third technology (Korean Unexamined Patent Application Publication No. 10-2005-0005823), an optical system for a head-mounted display that enlarges image light output from a predetermined display device at a predetermined magnification and produces large images at positions adjacent to both eyes includes a spontaneously emitted single light emitting display device for outputting predetermined image light, an X-prism equalizing the image light output from the light emitting display device, a pair of relay lens systems that enlarge, converge, and transfer each image light split and refracted by a reflective plane of the X-prism at a predetermined magnification, and a pair of reflective mirrors that are disposed adjacent to the left and right eyes of a user at a predetermined reflective angle so as to be able to convert and reflect the image light enlarged and converged by each relay lens system toward the left and right eyes.
Each of the prior arts includes the optical system for the HMD that enlarges the image produced from the display device and provides the enlarged image to the user. The optical systems have the half mirror and the reflective mirror as the common components.
Fundamentally, the half mirror should be located at a focal point formed by the reflective mirror. If this structure is not provided, various imaging lens units and relay lens systems should be introduced, and the image of the display device should be located at or inside the focus of the reflective mirror.
That is, the light reflected by the half mirror is enlarged on the entire plane of the reflective mirror so that an upright virtual image is formed. This structure can be miniaturized when a small display device having a size of 25 mm or less is used. However, a small display device has a disadvantage in that it is expensive if its resolution is increased. When an inexpensive and low-resolution display device is used, there is a limit in that the size of the HMD can be no longer reduced.
This imposes restrictions on realizing portability or miniaturization and is problematic in realizing a reduction in weight. This weight causes fatigue when the user wears the HMD on the head for a long time.
Further, the optical system for the HMD having such a structure generally has an angle of view of about 30 to 50 degrees, and thus reduces enlarging capability and creates difficulty providing a large image having a predetermined magnification. This can be overcome by reducing the radius of curvature of the reflective mirror. However, to do so, the half mirror should be disposed adjacent to the reflective mirror. In this case, the light path is interrupted, and thus it is difficult to provide a large image having a predetermined magnification.
FIG. 2 schematically shows an optical system using a principle of a magnifier among the conventional optical systems for the HMD. This structure can realize a large image using a display device having a size of 30 mm or more without using a small display device. However, a lens has a larger size than the display device, and thus the system has a very large size and is increased in weight. Thus, there is a disadvantage in that it is inconvenient to wear the HMD.